The need to reduce the emission of harmful byproducts and the ever-increasing pressures to increase fuel economy are ongoing challenges with internal combustion engines. Diesel engines produce an exhaust emission that generally contains at least four classes of pollutants that are legislated against by inter-governmental organizations throughout the world: carbon monoxide (CO), unburned hydrocarbons (HCs), oxides of nitrogen (NOx) and particulate matter (PM). A variety of emissions control devices exist for treating one or more of each type of pollutant. These emissions control devices are often combined as part of an exhaust system to ensure that all four classes of pollutant are treated before emission of the exhaust gas into the environment.
Various processes for the treatment of the exhaust gas proceed more rapidly when the NOx species in the exhaust gas comprises higher percentages of NO2 rather than NO. As a result, it may be desirable to provide an exhaust gas with higher percentages of NO2 to further boost NOx conversion in systems including such processes.